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Collecting data while drilling
No other technology used in petroleum-well construction has evolved more rapidly than measurement while drilling (MWD) and logging while drilling (LWD). Early in the history of the oil field, drillers and geologists often debated conditions at the drillbit. With advances in electronic components, materials science, and battery technology, it became technically feasible to make measurements at the bit, and transmit them to the surface so that the questions could be answered.
Directional measurements were the first measurements to have commercial application, with almost all use in offshore, directionally drilled wells. As long as MWD achieved certain minimum-reliability targets, it was less costly than single shots, and it gained popularity accordingly. The dual challenges of MWD and LWD technology were reliable operation in the harsh downhole environment and achievement of wireline-quality measurements.
In the early 1980s, qualitative measurements of formation parameters were introduced, often based on early wireline technology. Coring points and casing points were selected using short normal-resistivity and natural gamma ray measurements, but limitations in these measurements kept them from replacing wireline for quantitative formation evaluation. In the late 1980s, the first rigorously quantitative measurements of formation parameters were made. Initially, the measurements were stored in tool memory, but soon the 2-MHz resistivity, neutron porosity, and gamma density measurements were transmitted to the surface in real time. By the early years of the new millennium, there was a rapid expansion of the types of measurement available while drilling, including:
- Formation pressure
The terms MWD and LWD are not used consistently throughout the industry. Within the context of this section, the term MWD refers to directional-drilling measurements, and LWD refers to wireline-quality formation measurements made while drilling.
MWD and LWD system architecture
As MWD and LWD systems have evolved, the importance of customized measurement solutions has increased. The ability to add and remove measurement sections of the logging assembly as wellsite needs change is valuable, thus prompting the design of modular MWD/LWD systems. The following operational issues have become increasingly important in LWD systems:
- Fault tolerance
- Power sharing
- Data sharing across tool joints
- Memory management
The introduction of 3D rotary-steerable systems, which often use the same telemetry channel as the LWD systems, has reinforced the links between directional drilling and LWD.
A natural division in system architecture exists for drill-collar outside diameters (ODs) of 4¾ in. or less. Smaller-diameter tool systems tend to use positive-pulse telemetry systems and battery-power systems, and are encased in a probe-type pressure housing. The pressure housing and internal components are centered on elastomer standoffs, and mounted inside a drill collar. Some MWD/LWD systems are retrievable and replaceable, in case tool failure or tool sticking occurs. Retrievability from the drill collar while in the hole often compromises the system’s mounting scheme, and these types of systems are typically less reliable. Because the MWD string can be changed without tripping the entire drillstring, retrievable systems can be less-reliable, but still cost-effective, solutions.
For collar ODs greater than 6¾ in., LWD systems are often turbine-powered. When used with other modules, interchangeable power systems and measurement modules must supply power and transmit data across tool joints. Often, a central stinger assembly protrudes from the lower collar joint and mates with an upward-looking electrical connection as the collar-joint threads are made up on the drillfloor. These electrical and telemetry connections can be compromised by factors such as high build rates in the drillstring and electrically conductive muds. Recent MWD/LWD designs ensure that each module contains an independent battery and memory so that logging can continue even if central power and telemetry are interrupted. Battery power and memory also enable logging to be performed while tripping out of the hole. As the quantities of data gathered downhole increase, time spent dumping data on the rig floor becomes a significant factor affecting the economics of wireline replacement. Increasing efforts will be made to make the data-download process more rapid over the coming years.